1. Field of the Invention
The present invention relates to a fixing device in an image forming apparatus for fixing onto a sheet paper, by heating and pressurizing, a toner image formed in the image forming apparatus using an electro photographic process such as a copy machine, printer or facsimile, and a control method thereof.
2. Description of the Background
As a fixing device incorporated in an image forming apparatus using an electro photographic process such as a copy machine, printer or facsimile, there is a fixing device that fixes a toner image on a sheet paper by heating and pressurizing. In such a fixing device, a sheet paper is inserted between a pair of rollers including a heat roller and a press roller, or in a nipping part formed between similar belts, and a toner image is fixed thereon by being heated and pressurized. Meanwhile, a full-color toner image formed by superimposed toner images of plural colors has a toner layer thickness that is several times larger than that of a monochrome toner image.
Therefore, when the temperature of the heat roller is raised to melt the thick toner layer, only the surface of the toner layer melts excessively. If only the surface of the toner layer melts excessively, a high-temperature offset may occur. Moreover, for a color image, a difference in the way of melting of the toner layer causes a color difference in the fixed image and predetermined image quality cannot be acquired. That is, it is necessary to evenly melt the color toner layer and to prevent a color difference.
Thus, in a conventional, as a color image fixing device, there is a device that has a heating source both in a heat roller and a press roller, and heats both the heat roller and the press roller. This conventional fixing device can evenly melt a toner layer by heating the toner layer from both above and below. Also, the conventional fixing device can prevent a high-temperature offset of the toner.
However, also in such a conventional fixing device, heating control of the heat roller and the press roller is only on-off control of the heating source by application of predetermined power for each operation mode. Therefore, the heat roller and the press roller tend to have a temperature ripple and this temperature ripple may cause a color difference in the fixed image.
Meanwhile, high-speed warming-up is demanded in the fixing device in order to improve convenience for the user. However, if the total electrical quantity of a commercial power source that can be used in the entire color image forming apparatus can be 1500 W or less, no special installation work is necessary even in markets all over the world. Therefore, the electrical quantity that can be actually used for the heating source of the fixing device is the electrical quantity that remains after subtracting the electrical quantity used for a driving source such as a motor and the electrical quantity used for optional functions such as a finisher from 1500 W, and realization of high-speed warming-up is limited.
On the other hand, if the fixing device is pre-heated in the copy standby state to realize high-speed warming-up, power is consumed also in the copy standby state and energy saving in the image forming apparatus is hindered. Moreover, in the conventional fixing device, in which both the heat roller and the press roller have a heating source, the power consumption in the standby state may increase further.
Thus, in fixing a color toner image with a thick toner layer, by evenly melting the toner layer, it is possible to provide uniform image quality without generating a color difference in the fixed image. Moreover, a control method of the fixing device in the image forming apparatus is desired which enables energy saving without losing the convenience for the user.